Process for producing pigment particles of defined shape and size

ABSTRACT

A process for preparing pigment particles of defined size and shape comprises treating a sheetlike structure having through openings of defined shape and size with a polymerizable substance or mixture of substances in such a way that the openings are filled, removing any solvent present, polymerizing the substance or substance mixture, and isolating the resulting pigment particles from the openings. The pigments are readily varied in size and shape by variation of the substrate parameters and are useful for the known pigment applications, especially coatings, paints and inks.

DESCRIPTION

This invention relates to a process for preparing pigment particles ofdefined size and shape by treating a sheetlike structure having throughopenings of defined shape and size with a polymerizable substance ormixture of substances in such a way that the openings are filled,removing any solvent present, polymerizing the substance or substancemixture, and isolating the resulting pigment particles from theopenings.

Pigments are customarily prepared by precipitation reactions or bymechanical comminution of larger colored species. These methods ofpigment preparation give rise to pigment particles differing in shapeand size.

Prior German Patent Applications 19532419.6 and 19602795.0 describeprocesses for preparing pigments having a cholesteric liquid crystallineorder structure by printing processes. This process makes it possible toobtain pigment particles of uniform shape.

To manufacture high quality pigment coatings, especially those withinterference based color effects, it is advantageous to use pigmentparticles of defined uniform shape and size. However, obtaining asuitable thickness by printing is prohibitively complicated.

It is an object of the present invention to provide a process forpreparing pigment particles of defined shape and size where thethickness is easily adjusted.

We have found that this object is achieved by the process of thisinvention.

Suitable sheetlike structures having through openings preferably includeflexible structures such as wovens and especially nets. The sheetlikestructures can be formed for example from natural or synthetic threadsor metal wires, specifically from materials such as polyolefins,polyamides, polyesters or fluorinated polyolefins and wires composed ofin particular stainless steel, for example.

Particularly suitable sheetlike structures are nets suitable for screenprinting for example.

The mesh size determines the geometry of the pigment particles to beprepared, the size of these particles being advantageously within therange from 15 to 200 μm edge length, preferably within the range from 20to 100 μm edge length.

The treatment of the sheetlike structures with the polymerizablesubstances can be effected for example by dipping, knife coating orsaturating and squeezing off. It is also possible to apply thepolymerizable mixture to a support, for example a continuouslycirculating smooth support film, and then transfer it to a continuouslycirculating netlike film. The voids in the net and the polymerizablemixture become filled [sic]. It can be advantageous in this connectionto provide the sheetlike structures ahead of this treatment with agentswhich facilitate ready separation of the pigment particles from thesubstrate. Such agents include for example polyvinylpyrrolidones,vinylpyrrolidone copolymers, silicones, surface-active substances,long-chain fatty acids or esters or fluorosurfactants. The choice ofsheetlike structure also determines the release agents which are mostsuitable for the specific substrate. The polymerizable substances can beapplied as an aqueous dispersion or solution, for example.

It is advantageous to employ readily removable solvents, for exampletetrahydrofuran, dioxane, butyrolactone, esters such as methyl acetate,ethyl acetate or butyl acetate, ketones such as cyclohexanone, acetone,methyl ethyl ketone or diethyl ketone or hydrocarbons such as toluene.It is also possible to use halogenated alkanes such as methylenechloride, ethylene chloride, chloroform or carbon tetrachloride.

The removal of the solvent can be effected by evaporation at roomtemperature or elevated temperature, if necessary under reducedpressure. The temperatures have to be chosen so that unwanted thermalpolymerization does not occur.

Substances which can be converted into pigment particles are inparticular dyes and liquid crystalline compounds having polymerizablegroups, which become pigments on polymerization. However, it is alsopossible to use dyes or liquid crystalline compounds in combination withpolymerizable binders; on polymerization, the dyes or liquid crystallinecompounds then become embedded in the polymeric matrix.

The preparative process of this invention starts from polymerizableindividual compounds or from a polymerizable mixture. These startingmaterials can be for example organic or inorganic dyes. Either thesedyes are themselves polymerizable, for example through polymerizableside chains on the chromophores, or the dyes are mixed with apolymerizable binder, so that they become encapsulated in a polymericnetwork. The choice of dye depends on the later use of the pigment. Inprinciple, any dye can be used in the process of this invention. It isadvantageous to use dyes which are either insoluble or which can beincorporated covalently into the polymeric network.

Suitable dyes will now be more particularly described.

Suitable azo dyes are in particular mono- or disazo dyes, for examplethose having a diazo component derived from an aniline or from afive-membered aromatic heterocyclic amine which contains from one tothree hetero atoms selected from the group consisting of nitrogen,oxygen and sulfur in the heterocyclic ring and can be fused with abenzene, thiophene, pyridine or pyrimidine ring.

Important mono- or disazo dyes are for example those whose diazocomponent is derived for example from an aniline or from a heterocyclicamine of the pyrrole, furan, thiophene, pyrazole, imidazole, oxazole,isoxazole, thiazole, isothiazole, triazole, oxadiazole, thiadiazole,benzofuran, benzothiophene, benzimidazole, benzoxazole, benzothiazole,benzisothiazole, pyridothiophene, pyrimidothiophene, thienothiophene orthienothiazole series.

Of particular suitability are those diazo components which are derivedfrom an aniline or from a heterocyclic amine of the pyrrole, thiophene,pyrazole, thiazole, isothiazole, triazole, thiadiazole, benzothiophene,benzothiazole, benzisothiazole, pyridothiophene, pyrimidothiophene,thienothiophene or thienothiazole series.

Also of importance are azo dyes having a coupling component of theaniline, aminonaphthalene, aminothiazole, diaminopyridine orhydroxypyridone series.

The monoazo dyes are known per se and have been extensively described,for example in K. Venkataraman, The Chemistry of Synthetic Dyes, Vol.VI, Academic Press, New York, London, 1972, or in EP-A-201 896.

Anthraquinone, coumarin, methine and azamethine and also quinophthalonedyes can also be used with advantage.

Suitable anthraquinone dyes are described for example in D. R. Waring,G. Hallas, The Chemistry and Application of Dyes, pages 107 to 118,Plenum Press, New York, London, 1990.

Suitable coumarin dyes are described for example in Ullmann'sEncyklopadie der technischen Chemie, 4th edition, Volume 17, page 469.

Suitable methine or azamethine dyes are described for example in U.S.Pat. No. 5,079,365 and WO-A-9219684.

Suitable quinophthalone dyes are described for example in EP-83 553.

Polymerization means any type of reaction to build up polymers, ie.addition polymerizations in the form of chain reactions, additionpolymerizations in the form of stage reactions and also condensationpolymerizations.

As well as the dyes or interference colorants, the polymerizable mixturecan comprise various customary coating or printing ink additives such aspolymerizable binders, reactive diluents, dispersants, polymericbinders, fillers, thinners and also polymerization initiators.

Particularly suitable additives are polymeric binders and/or monomericcompounds which can be converted into a polymeric binder bypolymerization. Examples of suitable such agents are organic solventsoluble polyesters, cellulose esters, polyurethanes, silicones and alsopolyether or polyester modified silicones. Particular preference isgiven to using cellulose esters such as cellulose acetobutyrate.

Particularly suitable polymerizable substances contain reactivecrosslinkable groups such as acryloyl, methacryloyl, α-chloro-acryloyl,vinyl, vinyl ether, epoxy, cyanate, isocyanate or isothiocyanate groups.Monomeric agents are also suitable as binders, especially the reactivediluents well known in paintmaking, for example hexanediol diacrylate orbisphenol A diacrylate. Even small amounts of such substances--usuallyas little as 0.1-1% by weight--bring about a considerable improvement inthe flow viscosity. At the same time these agents have considerableinfluence on the mechanical properties of the cured pigment particles.

The polymerizable mixtures may further comprise polymerizationinitiators which decompose either thermally or photochemically and socause curing to take place. Preferred thermal polymerization initiatorsare those which decompose within the range from 20 to 180° C.,particularly preferably within the range from 50 to 80° C., to initiatethe polymerization. In principle, any photoinitiator can be used forphotochemical curing. More particularly, mixtures of various initiatorscan also be used to improve the degree of curing. Examples of highlysuitable photoinitiators are benzophenone and its derivatives, such asalkylbenzophenones, halomethylated benzophenones or4,4'-bis(dimethylamino)benzophenone and also benzoin and benzoin etherssuch as ethyl benzoin ether, benzil ketals such as benzil dimethylketal, acetophenone derivatives, such ashydroxy-2-methyl-1-phenylpropan-1-one and hydroxycyclohexyl phenylketone. Acylphosphine oxides such as2,4,6-trimethylbenzoyldiphenylphosphine oxide are particularly highlysuitable. Among the photochemically activable polymerization initiators,those which do not give rise to yellowing are preferred.

Particularly preferred polymerization initiators also include boronalkyl compounds and peroxides such as dibenzoyl peroxide anddi-tert-butyl peroxide.

The photoinitiators which, depending on the intended use for thepigments of this invention, are advantageously used in amounts withinthe range from 0.01 to 15% by weight, based on the polymerizablecomponents, can be used as individual substances or else--because ofadvantageous synergistic effects--in combination with each or oneanother.

Cationic polymerizations are preferably carried out using initiatorshaving charged structures. More particularly, substances are used whichare partly used in combination with acylphosphine oxides, for example:##STR1## and also derivatives of these compounds.

If desired, the polymerizable mixtures may also comprise stabilizersagainst UV and weather effects. Suitable for this purpose are forexample derivatives of 2,4-dihydroxybenzophenone, derivatives of2-cyano-3,3-diphenyl acrylate, derivatives of 2,2',4,4'-tetrahydroxybenzophenone, derivatives ofortho-hydroxyphenylbenzotriazole, salicylic esters,orto-hydroxyphenyl-s-triazines [sic] or sterically hindered amines.These substances can be used alone or preferably in the form ofmixtures.

Examples of contemplated fillers are rutile, anatase, chalk, talc andbarium sulfate.

Dispersants have a positive influence on the flow viscosity of thepolymerizable mixture and on the miscibility of the individual mixturecomponents. Any commercially available dispersant can be used.

Particularly suitable dispersants are those which are based on asuccinic imide, ester or anhydride structure as described in priorGerman Patent Application 19532419.6.

Contemplated polymerizable compounds also include interference colorantsin particular. Since the shape of such interference pigments has to meetparticularly high requirements, the process of this invention isparticularly suitable for preparing such pigments.

Particularly interesting interference colorants are cholesteric liquidcrystalline compositions.

Cholesteric liquid crystalline phases can be composed either of chiralliquid crystalline compounds or of achiral liquid crystalline compoundsadmixed with suitable chiral dopants.

The cholesteric liquid crystalline compositions used in the preparativeprocess of this invention preferably comprise the following components:

a) at least one chiral liquid crystalline polymerizable monomer or

b) at least one achiral liquid crystalline polymerizable monomer and achiral compound.

Particularly suitable components are in each case those components whichcan be converted or incorporated into a polymeric network via reactivegroups.

Suitable chiral liquid crystalline polymerizable monomers for use ascomponent a) include particularly those of the general formula I##STR2## where Z¹ is a polymerizable group or a radical bearing apolymerizable group,

Y¹, Y² and Y³ are each a chemical bond, oxygen, sulfur,--CO--O--,--O--CO--,--O--CO--O--, --CO--N(R)-- or --N(R)--CO--,

A¹ is a spacer,

M¹ is a mesogenic group,

X is an n-valent chiral radical,

R is hydrogen or C₁ -C₄ -alkyl,

n is from 1 to 6,

and the radicals Z¹, Y¹, Y², Y³, A¹ and M¹ can be identical ordifferent.

Preferred radicals Z¹ are:

H₂ C=CH--, HC.tbd.C--, ##STR3## --N═C═O, --N═C═S, --O--C.tbd.N, --COOH,--OH or --NH₂,

where the radicals R, which can be identical or different, each denotehydrogen or C¹ -C⁴ -alkyl such as methyl, ethyl, propyl, isopropyl,n-butyl, isobutyl, sec-butyl or tert-butyl. Of the reactivepolymerizable groups, the cyanates can trimerize spontaneously to formcyanurates and are therefore preferred. The other groups mentionedrequire further compounds having complementary reactive groups topolymerize. For instance, isocyanates can polymerize with alcohols toform urethanes and with amines to form urea derivatives. The same istrue of thiiranes and aziridines. Carboxyl groups can be condensed toform polyesters and polyamides. The maleiimido group is particularlysuitable for free-radical copolymerization with olefinic compounds suchas styrene. The complementary reactive groups can be present either in asecond compound of this invention, which is mixed with the first, orthey can be incorporated into the polymeric network by means ofauxiliary compounds containing 2 or more of these complementary groups.

Y¹ --Y³ each have the meanings mentioned at the outset, of which achemical bond is to be understood as meaning a single covalent bond.

Particularly preferred groupings Z¹ --Y¹ are acrylate and methacrylate.

Suitable spacers A¹ include all groups known for this purpose. Spacersgenerally contain from 2 to 30, preferably 2 to 12, carbon atoms andconsist of linear aliphatic groups. They can be interrupted in thechain, for example by O, S, NH or NCH₃, in which case these groups mustnot be adjacent. Suitable substituents for the spacer chain additionallyinclude fluorine, chlorine, bromine, cyano, methyl and ethyl.

Representative spacers are for example:

--(CH₂)p--, --(CH₂ CH₂ O)_(m) CH₂ CH₂ --, --CH₂ CH₂ SCH₂ CH₂ --, --CH₂CH₂ NHCH₂ CH₂ --, ##STR4## where m is from 1 to 3 and

is from 1 to 12.

The mesogenic group M¹ preferably has the structure

    (T--Y.sup.8).sub.s --T

where Y⁸ is a bridge member selected from the definitions for Y¹, s isfrom 1 to 3 and T is identical or different bivalent isocycloaliphatic,heterocycloaliphatic, isoaromatic or heteroaromatic radicals.

The radicals T can also be ring systems substituted by fluorine,chlorine, bromine, cyano, hydroxyl or nitro. Preferred radicals T are:##STR5##

Particular preference is given to the following mesogenic groups M¹ :##STR6##

Of the chiral radicals X of the compounds of the general formula I,preference is given, inter alia on grounds of availability, inparticular to those which are derived from sugars, binaphthyl orbiphenyl derivatives and also optically active glycols, dialcohols oramino acids. Suitable sugars are in particular pentoses and hexoses andderivatives thereof.

Examples of radicals X are the following structures, in which theterminal dashes denote the free valences in each case: ##STR7##

Particular preference is given to ##STR8##

Also suitable are chiral compounds having the following structures:##STR9##

Further examples are recited in German Application P 43 42 280.2.

R can be methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl,sec-butyl or tert-butyl as well as hydrogen.

n is preferably 2.

Of the achiral liquid crystalline polymerizable monomers mentioned underb), those are particularly suitable for the cholesteric liquidcrystalline composition which have the general formula II

    Z.sup.2 --Y.sup.4 --A.sup.2 --Y.sup.5 --M.sup.2 --Y.sup.6 --A.sup.3 --Y.sup.7 --Z.sup.3                                       II

where

Z² and Z³ are polymerizable groups or radicals containing apolymerizable group, Y⁴, Y⁵, Y⁶ and Y⁷ are each a chemical bond, oxygen,sulfur --CO--O--,--O--CO--,--O--CO--O--, --CO--N(R)-- or --N(R)--CO--.

Here the polymerizable groups, the bridge members Y⁴ to Y⁷, the spacersand the mesogenic group are subject to the same preference criteria asthe corresponding variables in the general formula I.

As well as an achiral liquid crystalline monomer, the cholesteric liquidcrystalline composition mentioned under b) comprises a chiral compound.

Preferred chiral compounds are those of the formula Ia ##STR10## whereZ¹, Y¹, Y², Y³, A¹, X and n are each as defined above and M^(a) is abivalent radical which contains at least one hetero--or isocyclic ringsystem.

The moiety M^(a) resembles the mesogenic groups described, since thisensures particularly good compatibility with the liquid crystallinecompound. However, M^(a) does not have to be mesogenic as such, sincethe only purpose of compound Ia is to bring about, through its chiralstructure, a corresponding twisting of the liquid crystalline phase.Preferred ring systems present in M^(a) are the abovementionedstructures T.

The polymerization for preparing the pigment particles is preferablycarried out photochemically. The light source depends on the nature ofthe polymerizable groups and also on the nature of any photoinitiatorused. Any illuminants usable in polymer technology are suitable.

As well as by exposure to light, the polymerization of the polymerizablemixture can also be brought about by means of electron beams or else,depending on the polymerizable groups, thermally.

After polymerization, the pigment particles can be isolated by variousmethods, for example by pulling the nets purely mechanically over asharp edge, in which case the pigment particles fall out of theopenings. In addition, compressed air, water or ultrasound can be usedfor separating pigment particles and substrate.

The pigment particles prepared by the process of this invention can havevarious shapes and sizes. A platelet structure is advantageousespecially for pigments whose coloring effect is based on interference.With these pigments, the perceived color is dependent on the viewingangle. The platelet structure permits uniform orientation of the pigmentparticles in the colored layer, giving rise to homogeneous reflection atmany pigment particles and leading to a homogeneous color.

A particular advantage of the process of this invention is thepossibility to prepare pigment platelets of identical shape and size. Asa result of the identical shape, the pigments of this invention,especially the interference pigments, exhibit a particularly brilliantcolor.

The pigments of this invention are useful as coloring constituents ofcoating compositions such as printing inks, emulsion paints and glosspaints. Such coating compositions may comprise further customaryadditives. Suitable additives are mentioned for example in prior GermanPatent Application 19532419.6, incorporated herein by reference.Coatings comprising the pigments of this invention are particularlysuitable for coating manufactured articles, especially vehicles such asautomobiles, motor cycles, etc.

In the Examples which follow, parts and percentages are by weight,unless otherwise stated. ##STR11##

EXAMPLE 1

A screen printing net from Estal (Estal MONO 40T) is filled with anisotropic solution of

    ______________________________________                                        53.8 g          of mixture A,                                                   3.4 g of compound B,                                                          0.5 g of cellulose acetobutyrate,                                             40.0 g  of butyl acetate and                                                  2.0 g of compound C.                                                        ______________________________________                                    

The butyl acetate solvent is evaporated off and the liquid crystallinefilm remaining behind in the screen is irradiated for 2 minutes under aUV laboratory lamp (220 V, 320 watt) in an atmosphere of nitrogen. Thecured film is subsequently broken out of the screen printing fabric.Microscopic examination of the resulting particles reveals a uniformsquare shape of the particles having an edge length of 160 μm. Theparticles exhibit a uniform green color in polarized light.

EXAMPLE 2

A screen printing net from Estal (Estal MONO 51T) is filled with anisotropic solution of

    ______________________________________                                        53.8 g          of mixture A,                                                   3.4 g of compound B,                                                          0.5 g of cellulose acetobutyrate,                                             40.0 g  of butyl acetate and                                                  2.3 g of compound C.                                                        ______________________________________                                    

The butyl acetate solvent is evaporated off and the liquid crystallinefilm remaining behind in the screen is irradiated for 2 minutes under aUV laboratory lamp (220 V, 320 watt) in an atmosphere of nitrogen. Thecured film is subsequently broken out of the screen printing fabric.Microscopic examination of the resulting particles reveals a uniformsquare shape of the particles having an edge length of 120 μm. Thethickness of the particles was 40 μm. The particles exhibit a uniformgreen color in polarized light.

EXAMPLE 3

A screen printing net from Estal (Estal MONO 55TX) is filled with anisotropic solution of

    ______________________________________                                        53.8 g          of mixture A,                                                   3.4 g of compound B,                                                          0.5 g of cellulose acetobutyrate,                                             40.0 g  of butyl acetate and                                                  2.3 g of compound C.                                                        ______________________________________                                    

The butyl acetate solvent is evaporated off and the liquid crystallinefilm remaining behind in the screen is irradiated for 2 minutes under aUV laboratory lamp (220 V, 320 watt) in an atmosphere of nitrogen. Thecured film is subsequently broken out of the screen printing fabric.Microscopic examination of the resulting particles reveals a uniformsquare shape of the particles having an edge length of 120 μm. Thethickness of the particles was 40 μm. The particles exhibit a uniformgreen color in polarized light.

We claim:
 1. A process for preparing pigment particles of defined sizeand shape, which comprises treating a sheetlike structure having throughopenings of defined shape and size with a polymerizable substance ormixture of substances in such a way that the openings are filled,removing any solvent present, polymerizing the substance or substancemixture, and isolating the resulting pigment particles from theopenings.
 2. A process as claimed in claim 1, wherein the sheetlikestructure used is a net composed of polymer or metal.
 3. A process asclaimed in claim 2, wherein the net used is composed of polyolefin,polyamide, polyester, fluorinated polyolefin or metal.
 4. A process asclaimed in claim 3, wherein the net used is suitable for screenprinting.
 5. A process as claimed in claim 1, wherein the polymerizablesubstance used is a liquid crystalline compound or dye havingpolymerizable groups or liquid crystalline compound or dye combined witha polymerizable binder.
 6. A process as claimed in claim 5, wherein theliquid crystalline compound used is a cholesteric liquid crystallinecompound or a nematic liquid crystalline compound doped with a chiralcompound.
 7. A process as claimed in claim 5, wherein photochemicallypolymerizable compounds are used.
 8. A process as claimed in claim 1,further comprising treating the sheetlike structure with an agent forfacilitating the isolation of the pigment particles from the throughopenings prior to the treatment with the polymerizable substances.